1. Field of the Invention
The present invention generally relates to write compensation methods and magnetic disk apparatuses and, more particularly, to a write compensation method and a magnetic disk apparatus for correcting a write signal for writing digital information.
Improvement in magnetic recording density in magnetic disk apparatuses is producing increasing small magnetization reversal intervals on a recording medium. Magnetization reversal for preceding recorded information (bit) on the recording medium causes a shift in the position of recording such that the position at which target information is written is displaced from an intended position of magnetization reversal. Such a shift may prevent the information from being read properly. This phenomenon is generally known as non-linear bit shift (described in detail later). Unless the non-linear bit shift is corrected, a displacement between the position of magnetization reversal and an actual write signal occurs.
2. Description of the Related Art
FIG. 1 is a block diagram showing a magnetic disk apparatus 1 according to the related art.
The magnetic disk apparatus 1 writes information on a rotatable magnetic disk 2 formed of a magnetic body such that a write current is supplied to a magnetic head 3 opposite to the magnetic disk 2 so as to generate a magnetic field in the magnetic head 3, thus magnetizing the magnetic disk 2. As the magnetic head 3 is driven by an actuator 4 to move radially (in the direction indicated by the arrow in FIG. 9) with respect to the magnetic disk 2, information is concentrically written on the magnetic disk 2. The magnetic head 3 is supplied with the write current from a head IC 5 so as to generate a magnetic field commensurate with the information to be recorded (hereinafter, simply referred to as target information).
The target information is supplied to a control circuit 6 from an outside apparatus. The control circuit 6 receives the target information and supplies the same to an encoder 7. The encoder 7 converts the target information supplied from the control circuit 6 into nonreturn to zero (NRZ) information to be written on the magnetic disk 2 and outputs the converted information. The output write signal from the encoder 7 is supplied to a write compensation circuit 8. A nonreturn to zero (NRZ) write method writes information using a binary signal pulse train in which an interval of a unit code coincides with an interval of a pulse.
The write compensation circuit 8 detects a bit stream of the output write signal of the encoder 7 and provides compensation in accordance with the pattern of the bit stream. The write signal corrected by the write compensation circuit 8 is supplied to the head IC 5. The head IC 5 supplies, to the magnetic head 3, the write current commensurate with the target information supplied from the write compensation circuit 8.
The signal picked up by the magnetic head 3 is supplied to the head IC 5 to be amplified thereby. The amplified signal is then supplied to an automatic gain control (AGC) circuit 9 which controls the signal supplied by the head IC 5 to have a regular amplitude and outputs the thus controlled signal.
The output signal of the AGC circuit 9 is supplied to a signal detection circuit 10 for detection of readout information. The readout information detected by the signal detection circuit 10 is supplied to a decoder 11 so as to be decoded thereby. The decoded signal is supplied to a control circuit 6 which outputs the reproduced information to an external device.
The output signal of the AGC circuit 9 is also supplied to a servo demodulation circuit 12 which processes the signal supplied by the AGC circuit 9 so as to demodulate a servo signal and supplies the servo signal to a servo control circuit 13. In accordance with the servo signal supplied by the servo demodulation circuit 12 and the control signal supplied by the control circuit 6, the servo control circuit 13 generates a drive control signal commensurate with a difference between a current position of the magnetic head 5 and a position at which recording or reading should occur. The resultant drive control signal is supplied to a drive circuit 14.
The drive circuit 14 generates a drive signal for driving the actuator 4 in accordance with the drive control signal supplied by the servo control circuit 13. The actuator 14 is driven by the drive signal from the drive circuit 14 so as to move the magnetic head 5 to a target position.
The magnetic disk apparatus 1 records information on the magnetic disk 2 by magnetizing the same. An inherent problem with such magnetic recording is that magnetization reversal for preceding recorded information (bit) on the recording medium causes a shift in the position of current recording so that the position at which target information is written is displaced from an intended position of magnetization reversal.
FIG. 2 shows how magnetization reversal occurs relative to a write signal in case no write compensation is provided. FIG. 2 in (A) shows an input NRZ signal carrying target information supplied from the encoder 7; FIG. 2 in (B) shows a write signal supplied to the magnetic head 3; and FIG. 2 in (C) shows magnetization occurring on the magnetic disk 2.
When no write compensation is provided, the target information supplied by the encoder 7 and the write current supplied to the magnetic head 3 are timed so as to be in phase.
It is assumed that the write current shown in (B) of FIG. 2 is used to magnetize the magnetic disk 2. When the polarity of the write current changes at time t0, the magnetic disk 2 is magnetized by a magnetic field at a polarity opposite to the preceding polarity.
When the polarity of the write current is reversed at time t2 subsequent to time t0 by a small time interval, the direction of magnetization on the magnetic disk 2 is reversed at a position that corresponds to time t1 preceding time t2 because the magnetic disk 2 is affected by the condition that occurred before time t0. Such a phenomenon is known as a non-linear bit shift. If the write operation is continued in this way, information continues to be recorded at positions displaced from respective intended positions of magnetization reversal, thus preventing the information from being properly reproduced from the magnetic disk 2.
For this reason, the magnetic disk apparatus 1 shown in FIG. 1 is provided with the write compensation circuit 8 between the encoder 7 and the head IC 5.
FIG. 3 shows how write compensation is provided in the magnetic disk apparatus according to the related art.
The write compensation circuit 8 according to the related art corrects a non-linear bit shift, that is, write precompensation (WPC) is provided by changing the position, on the recording medium, of magnetization reversal for target information depending on whether or not magnetization reversal for the immediately preceding bit (target information) occurs. Such a correction is intended to ensure that a readout operation properly picks up magnetization reversal.
More specifically, when magnetization reversal occurs successively, an interval preceding a write current reversal for a second bit and subsequent bits is extended as compared to a normal interval, so that magnetization reversal occurs normally on the recording medium.
Referring to FIG. 3, "1" indicates a case where magnetization reversal occurs for the immediately preceding bit; and "0" indicates a case where magnetization reversal does not occur for the immediately preceding bit.
As shown in FIG. 3, the write compensation circuit 8 provides a predetermined delay of t10 to the write current when magnetization reversal occurs successively for the past two bits.
FIG. 4 shows how magnetization reversal occurs relative to a write signal in case write compensation according to the related art is provided. FIG. 4 in (A) shows an input NRZ signal carrying target information supplied from the encoder 7; FIG. 4 in (B) shows a write signal supplied to the magnetic head 3; and FIG. 4 in (C) shows magnetization occurring on the magnetic disk 2.
Referring to (A) of FIG. 4, the input information requires a write current reversal at time t0 and then at time t1. That is, the input information requires that a write current reversal occurs for two successive bits. As shown in (B) of FIG. 4, the write current is reversed at time t2 with a predetermined delay of t10 from time t1, as described with reference to FIG. 3.
As shown in FIG. (C) of FIG. 4, the predetermined delay t10 in the write current causes a delay in magnetization on the magnetic disk 2 such that the magnetic disk 2 is magnetized at a position that corresponds to time t1 exactly when the input information requires a reversal. Thus, magnetization occurs at a position coinciding with reversal in recorded information (shown in (A) of FIG. 4) supplied from the encoder 7.
However, with the improvement in recording density in magnetic disk apparatuses, it is increasingly difficult to neglect an effect caused by magnetization reversal for the bit preceding the current bit (the bit to be recorded) by two bits.
When correcting magnetization reversal occurring for the bit preceding the current bit by two bits, the write compensation according to the related art may perform excessive compensation or insufficient compensation, thus producing an error in positions of magnetization reversal and a resultant failure in proper readout of data.
Referring to FIG. 4, the write compensation method according to the related art times the write current appropriately so that magnetization reversal occurs properly for successive two bits. However, the related art cannot provide proper write compensation when magnetization reversal is required for three successive bits. Referring specifically to (C) of FIG. 4, the magnetization direction that is to occur subsequent to time t3 is the same as the direction for the bit preceding the immediately preceding bit (that is, the bit preceding the target bit for writing by two bits). Accordingly, an effect from magnetization reversal for the immediately preceding bit is partially canceled. Since the related art described above causes the write current to be reversed at time t5 with the predetermined delay t10, the associated magnetization reversal occurs at a position that corresponds to time t4 displaced forward in time from time t3 at which the magnetization reversal should occur, ending up in a displacement between a time-dependent variation in the target information and corresponding positions of magnetization reversal.